fix(ctypes): only function descriptions have stubs

doc/changes/ctypes-no-stubs.md

@@ -0,0 +1,3 @@
+- ctypes: fix an error where `(ctypes)` with no `(function_description)` would
+  cause an error trying refer to a nonexistent `_stubs.a` dependency
+  (#9302, fix #9300, @emillon)

src/dune_rules/ctypes/ctypes_field.ml

@@ -276,3 +276,9 @@ let generated_ml_and_c_files ctypes =
   in
   ml_files @ c_files
 ;;
+
+let has_stubs = function
+  | None -> false
+  | Some { function_description = []; _ } -> false
+  | _ -> true
+;;

src/dune_rules/ctypes/ctypes_field.mli

@@ -73,3 +73,4 @@ val c_generated_types_module : t -> Module_name.t
 val type_gen_script : t -> string
 val c_generated_functions_cout_c : t -> Function_description.t -> string
 val function_gen_script : t -> Function_description.t -> string
+val has_stubs : t option -> bool

src/dune_rules/dune_file.ml

@@ -217,6 +217,10 @@ module Buildable = struct
   let has_mode_dependent_foreign_stubs t =
     List.exists ~f:Foreign.Stubs.is_mode_dependent t.foreign_stubs
   ;;
+
+  let has_foreign_stubs t =
+    List.is_non_empty t.foreign_stubs || Ctypes_field.has_stubs t.ctypes
+  ;;
 end
 
 module Public_lib = struct
@@ -823,9 +827,9 @@ module Library = struct
   let has_foreign_cxx t = Buildable.has_foreign_cxx t.buildable
 
   let stubs_archive t =
-    if List.is_empty t.buildable.foreign_stubs && Option.is_none t.buildable.ctypes
-    then None
-    else Some (Foreign.Archive.stubs (Lib_name.Local.to_string (snd t.name)))
+    if Buildable.has_foreign_stubs t.buildable
+    then Some (Foreign.Archive.stubs (Lib_name.Local.to_string (snd t.name)))
+    else None
   ;;
 
   let foreign_archives t = List.map ~f:snd t.buildable.foreign_archives

test/blackbox-tests/test-cases/ctypes/github-5561-name-mangle.t

@@ -20,15 +20,4 @@
   3 |     (module Type_description.Types)
                   ^^^^^^^^^^^^^^^^^^^^^^
   Error: Unbound module Type_description
-  File "dune", line 1, characters 0-211:
-  1 | (library
-  2 |  (name foo)
-  3 |  (ctypes
-  4 |   (external_library_name fooBar)
-  5 |   (build_flags_resolver vendored)
-  6 |   (generated_entry_point Types_generated2)
-  7 |   (type_description
-  8 |    (instance Type)
-  9 |    (functor Type_description))))
-  Error: No rule found for libfoo_stubs.a
   [1]

test/blackbox-tests/test-cases/ctypes/types-only.t

@@ -1,5 +1,5 @@
-When a ctypes description has just a type description, Dune does not setup a
-rule for a stubs file it tries to use.
+When a ctypes description has just a type description, if should be possible to
+build the library.
 See #9300.
 
   $ cat > dune-project << EOF
@@ -24,15 +24,3 @@ See #9300.
   > EOF
 
   $ dune build
-  File "dune", line 1, characters 0-185:
-  1 | (library
-  2 |  (name l)
-  3 |  (ctypes
-  4 |   (external_library_name none)
-  5 |   (build_flags_resolver vendored)
-  6 |   (type_description
-  7 |    (functor bindings)
-  8 |    (instance types))
-  9 |   (generated_entry_point c)))
-  Error: No rule found for libl_stubs.a
-  [1]